If you’ve ever gotten a flu shot, you may have felt soreness in your upper arm for a day or two afterward. That’s just a sign the vaccine is working, immunologists say.

The flu vaccine works by introducing your body to a foreign substance called an antigen (in this case, the antigen is a deactivated or “dead” virus similar to the flu virus). Being exposed to an antigen “primes” immune system cells, preparing them to create antibodies should they ever encounter the foreign substance again [PDF]. The inactive virus in the vaccine can’t make you sick, but it does help sensitize your immune system to potential threats. Essentially, the vaccine puts your body on high alert for anything resembling the flu virus, enabling you to fight off infections and illnesses faster and more effectively than you could otherwise.

In the process of battling the inactive flu virus introduced by the flu vaccine, your immune system also releases mediators like histamine, which causes inflammation. In the event of infection, inflammation is important because it helps your body fight invaders and repair damaged tissue. But it’s also what causes soreness. The flu shot is usually injected into your upper arm, which is why the early immune response—and any pain—tends to be localized there.

Roughly one in five people have this type of painful reaction, immunologist Richard Zimmerman told Popular Science. If you’re susceptible to soreness after receiving a flu shot, there are a few steps you can take to alleviate the pain. Dr. Juanita Mora of the American Lung Association recommends taking an ibuprofen about two hours before getting the shot.

“You can also try icing the injection site to reduce redness and swelling,” Dr. Mora said.

It’s also important to move your arm around after receiving the shot so the vaccine isn’t quite so concentrated in one place. Barring that, you could always try getting the vaccine in your non-dominant arm so that any pain won’t interfere with your everyday activities.

Any pain is worth it, though: Even if you’ve never gotten the flu before, there’s always a chance you could get it in the future. And getting a flu shot also contributes to herd immunity, helping protect higher-risk populations (like children, older adults, and chronically ill individuals) who often can’t receive the flu vaccine for themselves. Keeping up-to-date on vaccines is one of the easiest ways individuals can contribute to community health.

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